Quaslelastic light scattering has been used to detect and quantify non-invasively the molecular change associated with the early stages of cataract formation in the living human lens. It is proposed to develop a commercial QLS instrument based on specific improvements that have been made during this program to an existing prototype. The improvements include: 1) better measurement localization accuracy, 2) reduced measurement time, 3) utilization of a calibration standard, 4) easy to interpret measurement results based on a linear universal cataract-grading scale, and 5) use of a simpler and less expensive signal processor. There are many commercial applications for an instrument that provides a reliable non-invasive measurement of early cataractogenesis in vivo. To provide such a measurement capability has long been an important goal in human clinical cataract research. Such an instrument is needed for the following purposes: 1) To assess the effectiveness of putative anti-cataract reagents. 2) To evaluate the cataractogenic role of pharmacologic agents or radiation used in the treatment of systemic disease. 3) To characterize early cataract in epidemiologic studies of populations subject to differential cataractogenic stress. 4) To provide a quantitative basis for the decision to initiate surgical or medical treatment of cataract. Clinical studies will be conducted at one institution during the program.